Okay, clear this up for me

I understand that dark matter if it exists is some exotic material that I've never dealt with, but I have a couple of questions
Can I not see the dark matter because it is dark? If I had a pound of dark matter sitting on the table could I see it? could I touch it? could I see something behind a wall of dark matter?

I understand that dark matter if it exists is some exotic material that I've never dealt with, but I have a couple of questions
Can I not see the dark matter because it is dark? If I had a pound of dark matter sitting on the table could I see it? could I touch it? could I see something behind a wall of dark matter?

I believe that dark matter is thought to have exotic properties such as possibly only interacting via the gravitational force (hence, why it has only been 'detected' through it's gravitational influence). If that is the case, I suppose that the dark matter 'on' your table would pass through the table and accelerate towards the centre of the Earth (or, depending how much you have, both the Earth and the DM would accelerate towards each other), presumably settling in a simple harmonic oscillator type system (because of a lack of any frictional forces).
Of course, this is a highly speculative subject since the nature of dak matter is unknown and the above could well be way off the mark. For instance, if the source of the extra mass were neutrinos (as is one hypothesis), the above situation clearly wouldn't be at all plausible.
I hope that this is of assistance (and not strewn with errors, I am no authority on the subject).

thanks, I don't know if it helped much, but that is my fault. I'm just trying to get a mental picture of an exotic material. do you know if the dark matter has to be spread out thinly or could there be planet sized chunks of it? Didn't nutrinos get crossed off the list of possibilities or at least can only account for a small percentage of dark matter

Based on what is known about dark matter (very little), "chunking" seems highly unlikely. Ordinary matter forms solids and liquids due to electromagnetic force. Dark matter appears not to be subject to this force or anything else, except gravity.

Dark matter really is pretty exotic. For one, there is almost none of the stuff in our solar system. Planetary orbits are very nicely explained using plain vanilla newtonian gravity and using the measured mass of the planets [with a sprinking of asteroids, etc.]. Were there even a decent sized planets worth of 'dark matter' dispersed throughout our solar system, the outer planets would have some seriously screwed up orbits, or the theory of gravity would be as weird as quantum physics.

Review the work of Puthoff, Haisch, Rueda, et al and see if you are willing to contemplate that the particle-antiparticle virtual pairs of the quantum vacuum might fill the bill. They are practically indetectable except via the Lamb effect, the Casimir effect, and related properties. They exist for a very short time before annihilating, and the virtual particles are so bound by their shared debt that they must annihilate with one another (except perhaps when they can be separated a la Hawking Radiation, but that's another story). That satisfies the condition that the entity be invisible and weakly interactive. In the 1960's Sakharov posited that mass, gravitation, and inertia all arise from the interaction of matter with the quantum vacuum field. Hmmm.

Review the work of Puthoff, Haisch, Rueda, et al and see if you are willing to contemplate that the particle-antiparticle virtual pairs of the quantum vacuum might fill the bill. They are practically indetectable except via the Lamb effect, the Casimir effect, and related properties. They exist for a very short time before annihilating, and the virtual particles are so bound by their shared debt that they must annihilate with one another (except perhaps when they can be separated a la Hawking Radiation, but that's another story). That satisfies the condition that the entity be invisible and weakly interactive.

But gravitationally they should be very strongly interactive - about 10120 times that observed cosmologically.

Dark matter really is pretty exotic. For one, there is almost none of the stuff in our solar system.

Would there need to be? As long as dark matter is distributed basically evenly, with very little clumping (although it apparently does clump on galactic scales, forming dark matter halos around galaxies), I would think the density of dark matter would be within a few orders of magnitude of the density of ordinary matter in interstellar space, which is quite low--around 1 hydrogen atom per cubic centimeter according to this page. Anyone know what the average density of dark matter would have to be in the neighborhood of galaxies to explain the galactic rotation curves?

Can I not see the dark matter because it is dark? If I had a pound of dark matter sitting on the table could I see it? could I touch it? could I see something behind a wall of dark matter?

To start, let's clarify which theory of dark matter we're talking about. The most popular theories of dark matter involve weakly interacting massive particles (WIMPs). In the context of these models, the answers to your questions are:

1) No, you couldn't see it. If it interacts very little with normal matter, there is no way for it to produce photons for you to see. Similarly, it can't absorb light from some other source.
2) As has already been pointed out, it would pass right through the table, not being able to interact with the wood (or whatever).
3) If it can't interact with the table, it can't interact with your hand, so you wouldn't feel it.
4) Yes, it wouldn't absorb any of the light from the object behind it.

One of the other popular theories of dark matter involves black holes. If this were the case, the answers would be:

1) Depends on the mass of the black hole. A sizable one would noticably distort the light of objects behind it and accrete matter from its surroundings, producing light.
2) A sizable one would not be good for the table...
3) ...or your hand.
4) Yes, but it would be all distorted by gravitational lensing.

tribdog said:

do you know if the dark matter has to be spread out thinly or could there be planet sized chunks of it? Didn't nutrinos get crossed off the list of possibilities or at least can only account for a small percentage of dark matter

The black holes could be planet sized, but the WIMPs couldn't self-interact enough to produce clumps. Yes, neutrinos do make up only a small fraction of the dark matter.

Of course, this is a highly speculative subject since the nature of dak matter is unknown and the above could well be way off the mark. For instance, if the source of the extra mass were neutrinos (as is one hypothesis), the above situation clearly wouldn't be at all plausible.

In theory, that could happen with a neutrino (or any massive particle), but your intuition is right in the sense that it would be hard to arrange the energetics such that it became bound with the earth. Neutrinos are what we would call hot (or warm) dark matter because their typical velocities are near the speed of light. The current theories favor cold (low average velocity) dark matter because hot dark matter would have difficulty collecting itself into the dark matter "halos" that we see surrounding galaxies and clusters.

Would there need to be? As long as dark matter is distributed basically evenly, with very little clumping (although it apparently does clump on galactic scales, forming dark matter halos around galaxies), I would think the density of dark matter would be within a few orders of magnitude of the density of ordinary matter in interstellar space, which is quite low--around 1 hydrogen atom per cubic centimeter according to this page. Anyone know what the average density of dark matter would have to be in the neighborhood of galaxies to explain the galactic rotation curves?

Depends on what you mean by "in the neighborhood of". The average dark matter density in the universe is of order 10-6 cm-3. It rises as you get closer to the centers of galaxies and clusters, but still spans many orders of magnitude. Near the earth, it should have roughly the same mass density as the ISM (but that won't be true throughout the galaxy).

Neutrinos are what we would call hot (or warm) dark matter because their typical velocities are near the speed of light. The current theories favor cold (low average velocity) dark matter because hot dark matter would have difficulty collecting itself into the dark matter "halos" that we see surrounding galaxies and clusters.

The definition I am used to is that what determines if we call the dark matter "hot" or "cold" is wheter it is relativistic or not at freezeout! The reason why hot dark matter is favoured comes from structure formation in the early universe. Observations show that structures have been growing "from small clumps to bigger clumps", which is consistent with nonrelatvistic particles, and not from "large clumps splitting up into smaller clumps", which would be the case for relativistic particles. So in fact "hot" DM need not be relativistic right now, what matters is that it was at freezeout.

The definition I am used to is that what determines if we call the dark matter "hot" or "cold" is wheter it is relativistic or not at freezeout!

Well, I was trying to be pedagogical, but I've not seen a consistent definition of "hot dark matter" in the literature. It would seem the most useful definition of "hot dark matter" would be if it's relativistic when galaxy scale perturbations enter the horizon.

Well, I was trying to be pedagogical, but I've not seen a consistent definition of "hot dark matter" in the literature. It would seem the most useful definition of "hot dark matter" would be if it's relativistic when galaxy scale perturbations enter the horizon.

Yepp. That would be the most logical definition, but doesn't that almost coinside with freezeout...after freezeout the DM start building there potential wells, which ordinairy matter later can fall into.

Well, I was trying to be pedagogical, but I've not seen a consistent definition of "hot dark matter" in the literature. It would seem the most useful definition of "hot dark matter" would be if it's relativistic when galaxy scale perturbations enter the horizon.

I don't know if it answers your question, but this paper has a nice summary of different alternatives to the "cold, collisionless dark matter" model:

1. Strongly Self-Interacting dark matter (SIDM): The dark matter might have a
significant self-scattering cross-section [tex]\sigma[/tex], comparable to the nucleon-nucleon
scattering cross-section (46). Then, in any halo, large or small, where the
number of particles per unit area (the surface density) × [tex]\sigma[/tex] is greater than unity,
collisions amongst the dark matter particles leads to a complex evolution of
the structure. During the initial phases of this process, which lasts longer than
the present age of the universe, the central densities decline in the desired
fashion due to the scattering of dark matter particles. Also, scattering strips
the halos from small clumps of dark matter orbiting larger structures, making
them vulnerable to tidal stripping and reducing their number.

2. Warm dark matter (WDM): Dark matter may be born with a small velocity
dispersion (e.g., through decay of another species) (47, 48), which leaves it
now with only perhaps 100 m/s velocity but which can have a significant
effect on small scale structure. Extrapolating back in time, this velocity
increases to a value sufficient to have a significant effect on small-scale
structure (since the particles are moving too fast to cluster gravitationally on
these scales). There are fewer low mass halos and all halos have a less steep
profile in the innermost core. Also, because most of the lowest mass halos are
born by the fragmentation of larger structures in this picture, they are found in
high density regions and the voids tend to be emptier of small systems than in
the standard cold, collisionless dark matter scenario.

3. Repulsive dark matter (RDM): Dark matter may consist of a condensate of
massive bosons with a short range repulsive potential (49). The inner parts of
dark matter halos would behave like a superfluid and be less cuspy.

5. Self-Annihilating dark matter (SADM): Dark matter particles in dense regions
may collide and annihilate, liberating radiation (51). This reduces the density
in the central regions of clusters for two reasons: direct removal of particles
from the center and re-expansion of the remainder as the cluster adjusts to the
reduced central gravity.

6. Decaying dark matter (DDM): If early dense halos decay into relativistic
particles and lower mass remnants, then core densities, which form early, are
significantly reduced without altering large scale structure (52).

7. Massive Black Holes (BH): If the bulk of the dark matter in galactic halos
were in the form of massive black holes with mass of about one million solar
masses, then several dynamical mysteries concerning the properties of our
galaxy could be better understood (53). In normal galaxies dynamical friction
between the massive black holes and the ordinary matter would cause those in
the central few kiloparsecs to spiral into the center, depleting those regions of
dark matter and providing the ubiquitous central massive black holes seen in
normal galaxies.

It also discusses the different predictions that would be made by each of these models, and whether they could be made to fit with the observational evidence.

Yepp. That would be the most logical definition, but doesn't that almost coinside with freezeout...after freezeout the DM start building there potential wells, which ordinairy matter later can fall into.

Sure, they do occur at about the same time, but I don't think they're physically related.

JesseM said:

I don't know if it answers your question, but this paper has a nice summary of different alternatives to the "cold, collisionless dark matter" model

The model we're discussing has since been ruled out by observations, so it's not mentioned in that paper. The closest thing is "warm dark matter", which sits in between the hot and cold dark matter models. The term "hot dark matter" is often used loosely in the literature (much like the term "dark matter" itself), but the basic idea is the same -- high-temperature dark matter particles impede the growth of structure.

Yepp. That would be the most logical definition, but doesn't that almost coinside with freezeout...after freezeout the DM start building there potential wells, which ordinairy matter later can fall into.

The size of the horizon at the epoch at which the particles become non-relativistic fixes the smallest scale of the fluctuations surviving free-streaming damping. This smalles scale must be at least of galatic scale (or smaller) in order to fit with observations of matter distribution. So I would guess that the correct definition is the one that relates the transition to non-relativistic behaviour to the size of the smallest perturbations according to observations. You definition relates the transition to non-relativistic behaviour to the freeze out of that particles. If CDM particles may remain relativistic after a very early freeze out, but become non-relativistic before the horizon size is the required one to fit with observations, then both definitions are not equivalent. It it not very clear to me in which extent both definitions are actually equivalent for the currently postulated CDM.